The invention relates to a method of producing a porous plastic film, the method comprising producing a stretchable preform from a raw material blend comprising a polymer-containing basic material and an additive, stretching the blank so as to form a film comprising closed pores.
The invention further relates to a porous plastic film produced from a raw material blend containing a basic material and an additive mixed therewith, a plurality of pores being arranged in the structure of the plastic film.
Porous plastic films are known to possess a plurality of uses as packing material, sealing, thermal insulators, damp-proofs, sound-insulating material, basic material for printed matter and so on. Porous plastic films are also used because they save plastic material as compared with a solid film. Furthermore, the surface of a porous plastic film can be provided with a smooth and pleasant touch, which is a significant advantage in a plurality of applications.
One of the newest applications of a porous plastic film is an electromechanical film, an example being the product known as an EMFi film (Electro Mechanical Film). In an electromechanical-film, dynamic mechanical or acoustic energy generates an electric charge or a change therein, or vice versa, i.e. electric energy is converged into movement, oscillation or sound. Such a film is described in U.S. Pat. No. 4,654,546, for example.
An EMFi film is a thin, typically 30 to 100 μm thick, polypropylene film comprising enclosed cavities and acting as an electret. An electret refers to an object, herein a film, having a permanent electric charge and generating an electric field outside its surface provided the surface of the object is not electrically conductive. At present, a film is provided with a porous structure by biaxial stretching from a polypropylene plastic preform made for this purpose. The preform comprises a polypropylene (PP) that constitutes the matrix part of the film and with which calcium carbonate particles or another corresponding mineral filler is mixed. The particles of the mineral filler nucleate the breaking points or discontinuities of the matrix plastic, which during orientation cause a plurality of enclosed pores or cavities to be generated in the matrix part. After the stretching, the porous film is electrically charged for instance by direct current corona treatment, followed by metallization of at least one of the surfaces of the film.
An electromechanical film has been suggested for application for instance in microphones and loudspeakers, ultrasound detectors, hydrophones, electretic air filters, keyboards and operating switches, movement detectors, dynamic noise suppression applications, self-adhesive posters or the like, pressure, force and acceleration sensors, wind and rain detectors, location detecting floor surfaces and the like.
A problem associated with the production of known biaxially stretched porous films is that the generation of the desired fine-structured pore structure is extremely difficult with the most thermally stable plastics.
A further problem is that only few plastic materials are suitable for use as production material for films. Polypropylene (PP) is mainly used.
A still further problem associated with known porous electromechanical films is that their electromechanical constant (d33) decreases significantly and permanently when the temperature rises for a sufficiently long time, which restricts the temperature of use of the film to 50 to 60° C. depending on the application.